October 30, 2012

Weather Models Get Sandy Right

Anyone who ever attempted to plan around a five day weather forecast knows that long-range forecasts are accompanied by considerable uncertainty, and hurricane forecasts are no exception. Tropical cyclones routinely defy intensity forecasts and occasionally veer off of their projected paths, often by enough to leave residents placed under warning to wonder what the fuss was about. But Hurricane Sandy was a decisive win for the meteorologists and their computer models. Not only did the models correctly project Sandy's strength and path, they predicted an unprecedented storm--just about the highest accomplishment in forecasting.

Both Sandy's path and its sources of strength were highly unusual. Hurricanes typically draw their strength from warm tropical waters and are often pushed out to sea when they move north and encounter the prevailing westerly winds of the mid-latitudes. With Sandy initially moving into the Atlantic ahead of a cold front, hundreds of storms worth of precedent would dispose a forecaster to believe that the storm would have rapidly decayed encountered wind shear and moved over the colder waters of the northern Atlantic.

But while cold waters and strong upper-level winds usually combine to weaken tropical cyclones, Sandy actually intensified as it interacted with the same forces that usually destroy hurricane. A deep trough of low pressure and the subtropical jet-stream created enough wind shear to rip apart just about any other hurricane, but wound up creating enough baroclinic energy and upper level divergence to sustain and even intensify the cyclone despite colder waters and a hostile environment. Not only would this process transform Sandy from a typical, tightly wound tropical cyclone into a massive hybrid storm of unprecedented size and strength, but an unusual area of high pressure would prevent Sandy from moving out to sea and actually force Sandy back to the east and over the United States.

Despite the odds, it was as early as last Tuesday when the European model showed Sandy transforming into an extraordinarily intense hybrid storm and making landfall in the mid-Atlantic. Back then, Sandy was a small and utterly conventional tropical cyclone off the northern coast of Venezuela. Yet the European model peered 144 hours into the future and offered this prediction:

I actually laughed out-loud when I first saw this counter-intuitive forecast. Six day forecasts just aren't accurate and they occasionally produce far-fetched results. But over the next few days and before Sandy began to transform into the monster storm that would ultimately engulf the northeastern quadrant of the country, the other computer models joined one-by-one until the only disagreement was the exact location of landfall.

The models correctly anticipated an unprecedented storm with startling precision, nailing the storm's unusual path, strength, and character well in advanced. As predicted, Sandy transformed into a hybrid storm of unprecedented size and intensity, with tropical storm force winds stretching over 1,000 miles across, making it the largest tropical cyclone in the history of the Atlantic. While meteorologists often get a bad rap, they deserve credit for forecasting a historic storm well in advance.